Causes of Demand Ischemia
Demand ischemia occurs when there is an imbalance between myocardial oxygen supply and demand, resulting in inadequate oxygen delivery to the myocardium despite the absence of complete coronary occlusion. 1
Primary Causes of Demand Ischemia
Increased Myocardial Oxygen Demand
- Tachycardia/tachyarrhythmias - Increased heart rate reduces diastolic filling time and increases myocardial workload 1
- Hypertension - Elevated blood pressure increases left ventricular afterload and myocardial work 1
- Left ventricular hypertrophy - Increases myocardial mass and oxygen requirements 1
- Aortic valve stenosis - Creates pressure overload and increased ventricular wall stress 1
- Hypertrophic cardiomyopathy - Increases myocardial mass and oxygen consumption 1
- Fever - Elevates metabolic rate and cardiac output requirements 1
- Hyperthyroidism - Increases heart rate and contractility 1
- Arteriovenous fistula - Common in dialysis patients, increases cardiac output demands 1
Decreased Myocardial Oxygen Supply
- Anemia - Reduces oxygen-carrying capacity of blood 2, 3
- Hypoxemia - Decreases arterial oxygen content (e.g., from COPD exacerbation) 1
- Hypotension - Reduces coronary perfusion pressure 1
- Coronary microvascular dysfunction - Impairs myocardial perfusion despite patent epicardial vessels 1, 4
- Elevated ventricular filling pressures - Reduces the coronary perfusion gradient 1
- Coronary vasospasm - Transiently reduces coronary blood flow 1, 4
Pathophysiology of Demand Ischemia
Demand ischemia develops through a "perfect storm" of conditions that create an imbalance between oxygen supply and demand 1:
- Coronary blood flow may be compromised by low cardiac output or reduced diastolic blood pressure 1
- Elevated ventricular filling pressures worsen the situation by reducing the driving gradient for coronary perfusion 1
- Oxygen demand simultaneously increases due to elevated ventricular wall stress and neurohormone-mediated increases in heart rate and contractility 1
- This supply-demand mismatch results in subendocardial ischemia, as the subendocardium is most vulnerable to hypoperfusion 5, 6
Clinical Manifestations of Demand Ischemia
- Troponin elevation - Occurs due to myocyte injury even without epicardial coronary artery occlusion 1
- ECG changes - May show ST-segment depression or T-wave inversions rather than ST elevation 1
- Symptoms - May include chest discomfort, dyspnea, fatigue, or be asymptomatic 1
Common Clinical Scenarios Associated with Demand Ischemia
- Perioperative setting - Surgical stress, anemia, and hemodynamic fluctuations 3
- Sepsis - Combination of tachycardia, hypotension, and increased metabolic demands 1
- Gastrointestinal bleeding - Causes anemia and often tachycardia 1
- Acute exacerbation of COPD - Hypoxemia combined with increased work of breathing 1
- Severe aortic stenosis - Creates pressure overload with limited ability to increase coronary flow 1
Diagnostic Considerations
- Elevated cardiac biomarkers (troponin) may be present despite absence of acute coronary plaque rupture 1
- A rising and/or falling pattern of troponin is still important to distinguish acute from chronic elevations 1
- Evaluation should focus on identifying both the underlying cause of the supply-demand imbalance and any contributing coronary disease 1
Prevention and Management
- Address the underlying cause of increased demand or decreased supply 1
- Optimize hemodynamics (control heart rate, maintain adequate blood pressure) 1
- Correct anemia if present, especially in patients with known coronary disease 2, 3
- Consider beta-blockers to reduce myocardial oxygen consumption in appropriate patients 1
- Treat underlying coronary disease if present 1